1. Field of the Invention
The present invention relates to antenna devices, and particularly to antenna devices mounted in mobile communication terminal apparatuses, such as cellular phones, and to communication terminal apparatuses.
2. Description of the Related Art
Recently, as antenna devices mounted in mobile communication terminal apparatuses, casing dipole antennas have been proposed in which a metal body (for example, a ground plate of a printed wiring board) arranged within a terminal casing is used as a radiating element, as disclosed in Japanese Unexamined Patent Application Publication No. 2004-172919, Japanese Unexamined Patent Application Publication No. 2005-6096, and Japanese Unexamined Patent Application Publication No. 2008-118359. In a dipole antenna of this type, performance similar to that of a dipole antenna can be obtained by means of differential feeding for two casing ground plates (ground plate of a main body casing and a ground plate of a cover casing) in a folding-type or slide-type mobile communication terminal. Further, since the ground plates arranged in the casings are used as radiating elements, there is no need to provide separate dedicated radiating elements, resulting in a reduction in the size of mobile communication terminal apparatuses.
However, in the dipole antenna described above, the impedances of the ground plates vary in accordance with the shapes of the ground plates, the shapes of the casings, and states in which nearby metal objects (for example, nearby electronic components and hinge components) are arranged. Hence, to reduce the energy loss of a high-frequency signal, it is necessary to design an impedance matching circuit for each type of apparatus. Further, in a folding-type or slide-type mobile communication terminal apparatus there is a problem in that the impedances of ground plates and an impedance matching circuit vary in accordance with the positional relationship between the main body casing and the cover casing (for example, a state in which the cover is closed or a state in which the cover is open in a folding-type apparatus) and it is difficult to obtain a high gain in a state in which the casing is closed.